This invention relates to an improvement in ballast circuits and, more particularly, ballast circuits for controlling gaseous discharge lamps at low intensity or dim light levels.
Ballast circuits are, in general, used to regulate the amount of current through a gaseous discharge lamp having electrodes at each end. They primarily consist of impedance devices such as inductors, capacitors and transformers. Dimming circuits have been designed for use in conjuction with ballast circuits which are used to dim the lamp by allowing it to conduct only during a portion of each half cycle of input voltage. To simplify, the lamp is dimmed by turning on later in its current half cycle to obtain different light levels. It is desirable that the eye not see this turning off and on or a flicker will be observed. In studying auto-transformer type ballasts, having associated dimming circuits, it was noticed that flicker occurred at low light levels. There may be several causes of this instability.
Many prior art ballast circuits have one side, namely one of the heating windings, connected to ground through a resistor. The lamp electrodes are connected to the heating windings, and thus one of the electrodes is grounded. The ballast circuit as well as the case in which it is mounted should be grounded to reduce the hazard of electrical shock. However, when the ballast circuit is grounded through one of its heating windings, the lamp is noticed to flicker during low light intensity levels. When lamp current in the above arrangement is shown with an oscilloscope, a lack of symmetry of alternate half cycles has been observed by applicant. Applicant further has recognized that this lack of symmetry occurs because one electrode of the lamp is connected to ground while the other electrode is not. The main problem is that at low light levels, the lamp must be re-ionized for each half cycle. Since only one of the electrodes is grounded, through a resistor, one electrode will rise to a higher potential above ground than the other. The grounded side is voltage limited by this, whereas the ungrounded side is not. The ungrounded electrode which furnishes a large number of electrons on its negative half cycle while the grounded electrode frunishes a small number of electrons during its negative half cycle, bcause of the ground limitation. Ionization was, therefore, unequal during successive half cycles.
By using circuits incorporating this invention the ballast circuit is grounded and, at the same time, the attendant problems referred to above are overcome. A substantially symmetrical wave shape for each half cycle of current is secured and the flicker at low light levels is eliminated.